CMP is a combination of chemical reaction and mechanical buffing. A conventional CMP system includes a polishing head with a retaining ring that holds and rotates a substrate (also referred to interchangeably as a wafer) against a polishing pad surface rotating in the same direction. The polishing pad can be made of cast and sliced polyurethane (or other polymers) with a filler or a urethane coated felt.
During rotation of the substrate against the polishing pad, a slurry of silica (and/or other abrasives) suspended in a mild etchant, such as potassium or ammonium hydroxide, is dispensed onto the polishing pad. The combination of chemical reaction from the slurry and mechanical buffing from the polishing pad removes vertical inconsistencies on the surface of the substrate, thereby forming an extremely flat surface. However, conventional CMP and pad dressing methods have an important shortcoming—an uneven removal profile due to a lower polishing rate at the center of a wafer than at an edge of a wafer due to non-homogenous slurry distribution on the platen 110 during CMP.
As can be seen in FIG. 1A, during pad dressing to prepare a polishing pad on a platen 110 for CMP a pad dresser 100 is rotated in the same direction as the platen 110, e.g., clockwise, which holds the polishing pad. Similarly, during CMP, as seen in FIG. 1B, a polishing head 120 that retains a wafer (not shown) is rotated in the same direction as the platen 110—also clockwise. This leads to a lower removal profile at the center of the wafer than at the edge because of non-homogenous slurry distribution on the platen 110 surfaces and beneath the wafer during CMP. Specifically, more slurry is typically distributed at the edge of the wafer than at the center of the wafer causing more CMP to occur at the edges than at the center of the wafer. The non-homogenous distribution of slurry is possibly caused by the topography of the polishing pad, which is inclined in a direction that does not easily entrap and carry the slurry particles under the wafer. Further, as the CMP technology migrates to 300 mm wafers from 200 mm wafers, non-homogenous slurry distribution becomes more pronounced, as the slurry must travel an additional distance to reach the center area of the wafer, thereby worsening the lower removal profile at the center of the wafer.
Conventional systems and methods to correct this shortcoming generally include improved polishing head designs. However, these improved polishing head designs can be expensive, complicated, and difficult to control.
Therefore, a method is needed that overcomes the above-mentioned shortcoming without the expense, complications, and control issues related to improved polishing head designs.